@Finn dAbuzz,
Now you're just dancing. First because there
are historical records for the period to which i alluded, and second because you now insist on records from multiple sources. We don't have multiple sources for many events which are considered historically well-documented.
In 851, a large Danish force landed in Wessex, which was the Anglo-Saxon kingdom roughly south of the Thames. The king then, Ethelwulf, called out the
fyrd (militia) on a system devised by his father, with the majority of the forces going to the coasts of the Severn Sea and the "English channel" (an anachronism, as it wasn't called that at the time) to protect agains other landings, and then confronted the Danes with the main body of his troops, largely composed of the house carles, the men at arms, of the West Saxon nobility. He crushed the Danes in battle, inflicting heavy casualties on them. It would be 20 years before they attempted to invade Wesses again. The event is mentioned in passing in several sources, with no details, because it was extraordinary. Most of the time, when the Norse or the Danes invaded, the local nobility would club together and offer them a bribe to go away. It was rare that anyone would fight them, because if you weaken yourself in such warfare, when the Northmen are gone, your neighbors will fall upon you to take your land away from you. Bribery worked out well for all concerned. Ethewulf's victory was, therefore, extraordinary enough to get noticed.
Ethelwulf had five sons. His oldest son died before he did. When he died, he was succeeded by his second son, who died two years later. He was succeeded by Ethelwulf's third son, who died five years after that. At that time, Ethelwulf's fourth son, Ethelred succeeded to the throne. Ethelred came to the throne in the crucial year of 866.
In the winter of 865-66, two of the Ragnarson brothers, Ivar the Boneless and Ube, landed in East Anglia, planning to overwinter there. For the Danes and the Norse, such enterprises were venture capitalism in our terms. Neither Ivar nor Ube commanded more tha four or five ships, which is to say, somewhat over a hundred men each, so they had to get other ship masters and their crews "on board." They did this by promising plunder and paying them up front. Edmund, the king of East Anglia, negotiated rather then risking battle. (There is good inferential evidence that the Danes were contemptuous of those who negotiated--it is certain that they would soon be back to take everything you had, less what you'd already paid them.) He gave them horses, fodder for the horses, food for their ship's crews and a big chest of silver. With that, Ivar and Ube could pay their men again, and attract more ships to join them. All they had to do in return was promise to leave the churches alone (a favorite target of Northmen, since they usually had at least a little, and often a lot of gold and silver), and to leave in the springtime, which they intended to do anyway.
In 866, Ivar started north with ship's crews mounted on the horses Edmund had given them and others they stole along the way (after all, they hadn't promised not to steal horses). Meanwhile, Ube lead the bulk of their forces out to sea. The Northmen had a wonderful intelligence service, the "merchants" who visited all the lands around the North Sea (then known as the German Ocean). Ivar and Ube knew that there was civil war brewing in Northumbria. When Osbert, the ostensible king, learned of Ivar's invasion, he sent his main army south, and convinced his rival, Aelle (who may have been his brother, the sources aren't clear) to join him to repel the Danes. While Ivar engaged the Northumbrians, Ube landed and took the city which we now call York, thanks to the Danes. Osbert and Aelle united the forces of Northumbria, attacked York, and both were killed. The Danes then set up a puppet king.
There are only multiple sources for the event in two respects--one that the Anglo-Saxons just copied one another's work, usually not even bothering to change the wording. The other is a confused saga account which recounts the exploits of the sons of Ragnar Lothbrok ("Ragnar Hairy Pants"--almost certainly a mythical character). We know that Ivar, Halfdan and Ube (or Ubba) existed--there's no confirmation for either Ragnar or his other putative, heroic sons. So, for the invasion of Northumbria, there is actually only one source. It's by a monk, and so almost worthless politically, and completely worthless as a military account. The monks were fond of angels with flaming swords intervening when the Christians won, and demons attacking the faithful when the pagans won.
Ivar and Ube went on to overrun northern Mercia (the midlands) and returned to East Anglia, where Edmund was killed (and, of course, promptly made a saint). But unlike their previous raiding ways, the Danes had come to stay. Ivar and Ube went off to take and defend land in Ireland. (Always a dicey proposition, because the Irish would attack them relentlessly, for years on end. Ivar was killed in Ireland, and his brother Halfdan was killed attempting to avenge him.) Ube disappears from the Anglo-Saxon records for a decade.
In the winter of 870-71, Halfdan, the brother of Ivar and Ube, joined by Sigurd and Guthrum the Unlucky, invaded Wessex. Early in 871, Ethelred had lured them by concentrating all his stores in Abingdon (as it is now called) and the fighting them as Aesc's Hill (now called Ashdown), where he inflicted a heavy defeat on them. Ethelred may have been present in Mercia when the Danes had invaded--the king, Burghred, was Ethelred's brother-in-law. But we don't know it for a fact--it is inferred because after Burghred caved in to Danish demands (as local rulers usually did), the Danes did not attempt to advance any further. It is assumed that Ethelred was the reason, because the presence of an army is the only thing that would have stopped them, and the only army that could have opposed them would have been the West Saxons.
Ethelred died of wounds suffered in battle with the Danes in 871. He was replaced by the fifth and last son of Ethelwulf, his brother Alfred. Alfred, of course, has become the stuff of legends. But there is, in fact, only one contemporary source for his life, that of Bishop Asser. Asser was a Welsh monk, who became Alfred's friend (Alfred was disgustingly pious, if one is to believe Asser's account), and made Asser a Bishop. The only other records we have are the Anglo-Saxon chronicles. That is not a single record, but a compilaton of monastic records from many places. Usually, if it didn't happen in their neighborhood, the monks have nothing to say. Even when they recount events from further off, those account as suspiciously identical.
The Great Heathen Army, as the monks called the Danes and Norse who invaded "England" (a term not then in use) was
the major event of the 9th century, and the Danish presence continued to be the most important factor in the history of that island until early in the 11th century. It's great significance lies in the fact that the Northmen were not raiding, they had come to take land and to stay. Of course, whether raiding or invading, their motive was material gain.
You keep harping about historically documented--are you aware that there is only one contemporary source for the life of Charlemagne? Anselm was raised and then educated at Charlemagne's court in Aachen. The only other account of Charlemagne's life which is nearly contemporary is by a German monk called Notker the Stammerer. It's full of flying bishops and other absurdities, and bishops and monks are constantly getting the hapless and not very bright Charlemagne out of trouble. Do you consider Charlemagne to be not historically well-documented?
After the American civil war, the
Official Records of the War of the Rebellion was compiled, in 128 books, comprised in 70 volumes. A more comprehensive record cannot be imagined. Unlike modern conditions, in which orders and intelligence are transmitted by radio, and may never be written down--everything of that nature
was written down. And yet we still don't know the facts about crucial events, such as the wounding of Thomas Jackson at the battle of Chancellorsville. (The death of "Stonewall" Jackson three weeks later had a profound impact on the war--yet there is no single account that one can point to and say: "This is what happened.")
I've got news for you, Finn--your hammering on "historically documented" and insisting on multiple sources is really hilarious. But i will take it to heart, and never again waste my time giving you detailed answers to questions like yours.